1. Field of the Invention
The present invention relates to the field of maskless lithography. More particularly, the present invention relates to rasterization methods and modulation principles associated with pattern generation in a maskless lithography system.
2. Related Art
A significant challenge in maskless lithography is the development of efficient rasterization techniques to configure a reticle for projecting a desired pattern onto a substrate. For example, in the case of a spatial light modulator (SLM), the rasterization technique determines how to configure the SLM pixels in a manner that will result in projection of the desired pattern through the lithography system's projection optics.
One conventional rasterization technique used to configure SLM pixels includes describing the desired pattern as a union of polygons. For example, if the desired pattern is an integrated circuit (IC), the IC will be first described as the union of polygons that represent the elements of the transistors, logic devices, or other elements that combine to form the IC. These polygons include the lines, edges, and sequences of lines that form the smallest segments of the IC elements. This conventional rasterization technique is then used to select states of the SLM pixels to create images of these IC elements.
This conventional rasterization technique includes the computation of a parameter illumination table based on an isolated edge parallel to the pixel grid. This illumination table approach, however, has several limitations. First, it does not take into account the orientation of the edge. For example, edges associated with the desired pattern may have different slopes with respect to the pixel grid. Secondly, the conventional illumination table approach fails to take into account the interaction between edges of the pattern in the case when they are in close proximity to each other.
The limitations of the conventional illumination table approach, noted above, ultimately produce distortions in the aerial image. These distortions subsequently restrict the pattern resolution and the pattern placement accuracy that can be achieved by the lithography system.
Rasterization techniques are generally flexible enough to account for anomalies associated with different types of modulation. For example, rasterization values for configuring SLM pixels associated with tilting mirrors will differ from the values required to configure pistoning mirrors. This flexibility, however, is insufficient to compensate for more subtle anomalies. These more subtle anomalies can include imperfections in individual mirrors, such as curling and mirror height variations, and can lead to phase errors causing through-focus dose uniformity problems.
Tilt mirrors cannot inherently resolve these dose uniformity problems. For example, through-focus dose uniformity problems cannot be resolved by the absence of actuation of a piston degree of freedom (as is the case in pure tilt mirrors). Another challenge in relying on tilt mirrors, in these applications, is that tilt mirrors can only represent 0° or 180° phase intensity. For special situations (e.g., vortex contact hole printing), other phases (e.g. 90° or 270° or even intermediate values 45°, 135°, 315°) are needed.
Traditional pistoning mirrors alleviate some of the through-focus dose uniformity problems noted above. However, traditional pistoning mirror techniques can have parasitic tilt effects (that can be constant vs. phase or can vary with the phase actuation). Parasitic tilt leads to amplitude variations and finite values of the derivative of phase vs. the x or y coordinate, in turn causing not only through-focus dose variations, but also causing telecentricity errors.
What is needed, therefore, is a more efficient rasterization method and system for configuring SLM pixels to project desired patterns onto a substrate. What is also needed is a method and system to compensate for imperfections in individuals mirrors of the SLM.